The present invention concerns, in general terms, the monitoring of the quality of a liquid medium and in particular more or less salty water in coastal areas or estuaries.
More precisely, the invention concerns a system for measuring or monitoring the quality of a liquid medium comprising first and second points for taking off a sample from the liquid medium and a hydraulic circuit in which the samples taken from the first and second sample-takeoff points circulate. The hydraulic circuit comprises in particular                a measuring chamber provided with at least one measuring sensor able to measure a quantity characteristic of the liquid medium, such as a physicochemical or biological parameter of the liquid medium, said measuring chamber having a first opening in fluid communication with the first sample-takeoff point and a second opening, and        a first suction pump having a suction inlet connected to the second opening of the measuring chamber and a discharge opening in fluid communication with the liquid medium, the first suction pump being able, in operating configuration, to circulate in the hydraulic circuit a sample taken from the first sample-takeoff point.        
A measuring or monitoring system of this type is for example described in the patent application FR 2 741 446.
To function correctly, this system is supplemented by a de-aerator placed above the first pump in order to trap the gases present in the circuit when the system is started up as well as the gases issuing from the natural degassing of the sampled liquid. It also comprises a priming pump connected to the de-aerator in order to prime the hydraulic circuit at the start of operation and a sample-takeoff system for selectively connecting the sample-takeoff points to the inlet and outlet of the hydraulic circuit.
Although giving complete satisfaction with regard to its performance, this measuring system has a few characteristics that could be improved, such as relatively high structural complexity and energy consumption.